1. Field of the Invention
This invention generally relates to wireless communications and, more particularly, to a system and method for reducing the effects of clock generated harmonics in the transceiver passband of a wireless communications device.
2. Description of the Related Art
Processors, such as microprocessors, are an integral part of many electronic products, including a typical wireless communications device. Microprocessors are typically used to execute instruction sets and to generally control the operation of devices. A clock is used with the typical microprocessor to provide a frequency at which the microprocessor operates. A crystal is a conventional and inexpensive way of supplying a microprocessor reference frequency. As is well known, spurious frequencies such as harmonics, are generated by the clock, along with the desired clock frequency. These spurious frequencies, as well as the crystal resonant frequency, “leak” through to other circuits on the device printed circuit board through conductance on the power lines or grounds, and through radiation. These spurious frequencies can interfere with the operation of a device by manifesting themselves as noise to other portions of the wireless communications device.
A wireless communication device transceiver circuit is especially susceptible to interference from spurious signals. In some instances, the spurious signals have frequencies equal to the carrier frequency of the wireless communications device. By equal to the carrier frequency, it is to be understood that this includes spurious signals that have some power in the passband of the wireless communication device. These spurious signals may provide undesirable noise and degrade performance of the device. For example, noise in the passband of a receiver can degrade sensitivity, while noise in a transmitter passband can violate system specifications, or raise the general noise floor of the system in which the wireless device is operating.
To solve spurious signal and interference problems, it is typically necessary to add filtering circuits or shielding to the circuit board. However, this solution takes time and adds to the total parts count. In some instances, the size and position of the board may limit the filtering that can be provided. It is known to change the clock frequency to avoid harmonic interference with the local oscillator or other transceiver circuitry. However, it is not desirable to reselect wireless device crystals to accommodate printed circuit board filtering issues. In wireless devices that operate with more than one transceiver frequency, it may be difficult to find a single clock frequency that supports the operation of the digital circuitry, and yet avoids interfering with at least one of the transceiver frequencies.
It would be advantageous if the undesirable effects of spurious frequencies, such as those harmonically related to a clock, could be eliminated from or substantially reduced in the passband of a transceiver without special filtering.
It would be advantageous if a wireless communications device could be operated at a plurality of transceiver frequencies while reducing the noise effects of spurious signals related to clock generated harmonics.